Regulatory effects of ectomycorrhizal fungi on the morphological construction of Pinus sylvestris var. mongolica seedlings under exogenous nitrogen input

• ECMF boosts bacterial diversity and P availability under low N. • High N causes C N imbalance, weakening mycorrhizal benefits. • Mycorrhiza induces N-dependent root and shoot plasticity. • N addition is the key driver of seedling morphology via cascading effects. This study investigated how ectomycorrhizal (ECM) fungal inoculation affects the rhizosphere microenvironment and growth of Pinus sylvestris var . mongolica seedlings under different nitrogen (N) addition levels. Results showed microbial diversity and biomass exhibited threshold responses to N. Under ECMF inoculation, low N (LN) increased bacterial diversity (Chao1 index +13.40 %) and microbial biomass phosphorus (+139.20 %), whereas high N (HN) only marginally increased diversity and decreased microbial biomass carbon (-41.4 %). LN inoculation also increased total soil C and P, alleviated microbial C limitation, and promoted seedling morphology (e.g., specific root length +58.97 %, seedling height +58.58 %). In contrast, HN inhibited root development and exacerbated the effects of N limitation. Without inoculation, HN resulted in higher bacterial diversity and soil nutrient content than did LN. Path analysis identified N addition and inoculation as key positive drivers of aboveground growth, with root morphology showing negative feedback. In conclusion, ECMF inoculation benefits seedling growth by enhancing microbial diversity and P availability under low N conditions, but high N causes C N imbalance and suppresses mycorrhizal benefits, providing theoretical support for mycorrhizal application in semi-arid areas under N deposition. Consequently, based on an identified threshold of ≈6 g·m⁻²·a⁻¹ for N addition, we recommend ECMF inoculation only under low-N conditions to ensure seedling morphological formation. This practice should be avoided under high-N conditions to prevent the loss of symbiotic benefits and a decline in ecological adaptability.